A fuel cell is composed of modules stacked as many as necessary. Each of the modules is composed by joining an electrolyte membrane causing power generating reaction with catalyst layers, and providing separators interposing the joined material in between. In order to seal a fuel gas, the electrolyte membrane is fixed to a frame that is usually made of resin, by injection molding of a fixing seal referred to as a gasket preventing the fuel gas from leaking. The electrolyte membrane fixed to the frame is provided between the separators, and the assembly is referred to as a module.
In recent years, a proton conductive ion-exchange membrane has been used as the electrolyte membrane for a polymer electrolyte fuel cell. In particular, perfluorocarbon polymer (hereafter referred to as sulfonic perfluorocarbon polymer) is widely considered as its superior basic characteristics. One of the requirements for an actual electrolyte membrane used for a fuel cell is a low ohmic loss of the membrane. The ohmic loss of the membrane depends on the conductivity of an electrolyte polymer used for the membrane.
Methods for reducing the electric resistance of a positive ion exchange film include increasing the concentration of sulfonate acid group and reducing the thickness of the membrane. However, a significant increase in the concentration of sulfonic acid group reduces a mechanical strength of the membrane and results in a creep in the membrane when operating the fuel cell for a long time, causing a problem of reduced durability of the fuel cell, for example.
In addition, an electrolyte membrane having high concentration of sulfonic acid group significantly swells when absorbing moisture, and it tends to cause various disadvantages. For example, the dimensions of the membrane are likely to increase due to moisture generated at the time of power generation reaction, water vapor supplied with the fuel gas, and others. The increase in the dimension of the membrane contributes to forming “crinkles” in the membrane, and the “crinkles” fill grooves in separators, inhibiting flow of gas. Furthermore, by repeatedly stopping operation, the membrane swells and shrinks repeatedly. With this, the membrane or the electrodes fused to the membrane cracks, reducing the characteristics of the cell.
Providing a reinforcement layer in an electrolyte membrane has been proposed as a technique for solving the problem described above (see PTL 1 to 7).
As illustrated in FIG. 12, PTL 1 discloses a membrane-electrode assembly having solid-polymer electrolyte membrane 111 and porous sheet 113 provided as a reinforcement layer in electrolyte membrane 111. According to the configuration of membrane electrode assembly in PTL 1, porous sheet 113 is present at a center part in the thickness direction of electrolyte membrane 111, and reduces the change in dimension in the in-plane direction.